Not applicable.
Not applicable.
1. Field of the Invention
The present invention relates generally to current mirror circuits, and more particularly to a low voltage source follower output stage that bootstraps the output impedance of the mirror driving it so that changes in input supply voltage and load current have substantially less effect on the output voltage.
2. Background of the Invention
xe2x80x9cBootstrappingxe2x80x9d is a term of art in electronics, and is used to increase the output impedance of a current mirror, thereby increasing open loop gain and providing more closed loop accuracy as well as improved power supply rejection. Bootstrapping is commonly accomplished by driving a common circuit node so that the common circuit node voltage maintains a constant relationship to an output of the circuit. Bootstrapping also commonly requires additional supply current to bias the driver circuit.
Current mirrors are commonly used in operational amplifier circuits so that a single reference current may be used to generate additional currents referenced to each other throughout the circuit. A current mirror circuit may generally include a configuration such as a transistor, having its base and collector short-circuited, and connected at two points to a second transistor. The connection between the first transistor and the second transistor is base-to-base and emitter-to-emitter.
In U.S. Pat. No. 5,592,123 (xe2x80x9cthe ""123 Patentxe2x80x9d) issued to Ulbrich on Jan. 7, 1997, discloses a floating current mirror circuit for achieving high open loop gain without additional voltage gain stages. According to Ulbrich""s disclosure, this invention avoids additional frequency compensation and increased power dissipation. FIG. 1 illustrates the invention described in the ""123 Patent featuring a current mirror bootstrap for increasing the output impedance of the current mirror by driving the common node 36 of the current mirror with emitter follower transistor 30. The current mirror differential input 39 is applied from a differential amplifier 37. By increasing the output impedance of a current mirror at the output of an amplifier stage, the open loop gain is increased thereby providing more closed loop accuracy as well as an improved power supply rejection ratio. The uncorrected bootstrap error is the difference in collector-emitter voltage between transistor 28 and transistor 26 that form the current mirror. This voltage is also the difference between node 34 and node 35 voltages which is proportional to 1/gm of transistor 30, where gm=[qle/(KT)], and Ie=(current source 24)xe2x88x92(current source 21)xe2x88x92(current source 22). While clamping node 34 with the Vbe of transistor 30 provides beneficial increase in mirror output resistance, it limits the voltage swing available to drive an output stage that follows. The voltage swing Vsw=Vnode32xe2x88x92Vbexe2x88x92V22xe2x88x92V24xe2x88x92V38, where Vnode32 is the positive supply voltage, Vbe is the base-emitter voltage of transistor 30, V22 is the voltage across current source 22, V24 is the voltage across current source 24, and V38 is the supply voltage common.
There is a need for a circuit that provides improved reference output circuit accuracy while accommodating both low output voltage and low supply voltage. There is also a need for a circuit that provides stable capacitive load drive capability at low supply quiescent current. There is also a need for a circuit that provides greater bootstrap output voltage swing without increasing the quiescent(unloaded) power dissipation of the circuit.
The present invention solves the needs addressed above. The present invention provides a circuit that includes a signal current mirror, a source follower output transistor, a sense transistor, and an output current mirror. This circuit has improved performance due to the source follower transistor, the sense transistor and the output current mirror, these items forming a common source difference amplifier. This common source difference amplifier adjusts the common voltage of the signal mirror to keep equal voltages at the input and output of the signal mirror. The voltage swing available at the output of the signal current mirror is increased over the prior art by Vbexe2x88x92Vsat, where Vbe is the base-emitter voltage and Vsat is the minimum collector-emitter voltage of a bipolar transistor operating in the forward active mode. Thus, the common node of the mirror adapts to changing supply voltage, output load current and temperature so that the effect on output voltage is minimized. For optimum performance, the current density ratio of the output current mirror devices is equal to the current density ratio of the sense transistor to the source follower transistor.
The present invention uses a source follower output stage which is not used in the prior art. This source follower output stage provides advantages over the prior art, including lower output impedance to minimize output voltage change with changing load current as well as improved stability driving capacitive loads. Capacitive load drive capability is proportional to the source follower gate to common(ground) capacitance.
The prior art also does not include connecting two mirrors as in the present invention. The present invention uses an output mirror to bootstrap the signal mirror. This configuration provides increased voltage swing at the signal mirror output allowing a minimum output voltage Vbexe2x88x92Vsat (or VT for MOSFETs) lower than the prior art. This may amount to a 400 mV output voltage reduction allowing for a 1.6V output voltage from a 1.8V (two battery) supply instead of 2.0V output voltage from a 2.7V (three battery) supply. An additional benefit is that the output mirror current is proportional to sinking load current for high efficiency. When the sinking load current is small, the output mirror current is low.
It is an object of the invention to provide improved circuit performance by boosting the output impedance of a current mirror so that changes in input supply voltage and load current have substantially less effect on output voltage.
It is also an object of the present invention to provide bootstrap accuracy at lower output and supply voltage without requiring a higher quiescent current.
It is further an object of the present invention to provide a circuit for use with varying output loads and capacitive loads.
The benefits of the present invention make the invention very useful in a number of applications. Those applications include battery-powered applications where as few batteries as possible are desired. Portable electronics, including CD players and cellular phones, would be benefited by aspects of the present invention.